Light-triggered reversible "one-to-two" morphological transition in a "latent double-amphiphilic" linear-hyperbranched supramolecular block copolymer

Article abstract of DOI:10.1039/c6cc03445d

This study reports a new category of stimuli-responsive morphological transitions, i.e., from one morphology (e.g., vesicles) to another two different ones (e.g., nanosheets and nanofibers), by investigating the light-responsive self-assembly behaviour of a "latent double-amphiphilic" linear-hyperbranched supramolecular block copolymer.

Full text of DOI:10.1039/c6cc03445d

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Light-triggered reversible ‘‘one-to-two’’
morphological transition in a ‘‘latent
double-amphiphilic’’ linear-hyperbranched
supramolecular block copolymer†
Cite this:DOI: 10.1039/c6cc03445d
Received 25th April 2016,
Accepted 13th May 2016
Wenfeng Jiang,a Yong Liu,a Chunyang Yu,a Shanlong Li,a Yongjin Li*b and
DOI: 10.1039/c6cc03445d
Yongfeng Zhou*a
www.rsc.org/chemcomm
This study reports a new category of stimuli-responsive morpholo- fewer examples took a more complicated mode: from A to B and
gical transitions, i.e., from one morphology (e.g., vesicles) to another then to C with or without intermediates during the transition
two different ones (e.g., nanosheets and nanofibers), by investigating process (shown as B).6 The common characteristic in category II
the light-responsive self-assembly behaviour of a ‘‘latent double- is that there is only one new morphology generated in one transition
amphiphilic’’ linear-hyperbranched supramolecular block copolymer. step, which can be summarized as ‘‘one-to-one’’ morphological
transition.
Self-assembly of amphiphilic block copolymers in selective
To our knowledge, up to now, the reported stimuli-responsive
solvents has drawn considerable attention for decades. A charming morphological transitions in block polymer self-assembly are
part of it lies in stimuli-responsive self-assemblies.1 A phenom- limited to ‘‘one-to-zero’’ and ‘‘one-to-one’’ transitions. Another
enon usually involved is the morphological transition of self- logically reasonable transition from one (A) morphology to the
assemblies in response to environmental stimuli. Hitherto, the other two (B plus C) or more morphologies, that is, the transition
reported morphological transitions can be divided into two from ‘‘one to two or more’’ (category III in Table 1) has not
categories (Table 1) based on the transition modes. In category I, been disclosed. Herein, we report for the first time on a light-
morphological transitions take a mode from morphology A to responsive ‘‘A to (B plus C)’’ reversible morphological transition
unimers in an irreversible or reversible way. This kind of transi- from a novel amphiphilic linear-hyperbranched supramolecular
tion is quite common in the self-assembly of double-hydrophilic block copolymer of polyethylene glycol-b-hyperbranched poly(3-
block copolymers (DHBCs),2 amphiphilic block copolymers with ethyl-3-oxetanemethanol) (PEG-b-HBPO). Scheme 1 illustrates the
blocks connected by dynamic covalent bonds such as disulfide overall transition both from molecular and morphological levels.
bonds,3 and dynamic non-covalent bonds such as host–guest Initially, two amphiphiles of AZO-PEG (azobenzene terminated
interactions.4 As this process involves only one kind of regular PEG) and CD-g-HBPO (b-cyclodextrin grafted HBPO) couple
morphology A to its disassemblies of unimers, it can be termed as together to form the supramolecular block copolymer of PEG-b-
a ‘‘one-to-zero’’ morphological transition. In category II, one type HBPO under visible light. PEG-b-HBPO is amphiphilic and self-
of self-assemblies (type A) will transit into another type (type B) in assembles into vesicles in water. Under UV irradiation, however,
an irreversible or reversible way, and the intermediates of [A + B] PEG-b-HBPO is conversely dissociated into AZO-PEG and CD-g-
will sometimes appear during this transition (shown as A).5 For HBPO, and meanwhile, the vesicles disappear and transit into
example, O’Reilly and co-workers reported a diblock copolymer a mixture of nanofibers from AZO-PEGs and nanosheets from
of poly(methyl acrylate)-b-poly(N-isopropyl acrylamide) (PMA-b- CD-g-HBPOs. This represents a light-responsive ‘‘one to two
PNIPAM) underwent a fast and reversible transition from or more’’ morphological transition. This transition is totally
micelles (A) to vesicles (B) by raising the temperature. Much reversible, and the nanofibers and nanosheets can transit
a School of Chemistry & Chemical Engineering, State Key Laboratory of Metal
Table 1 Stimuli-responsive morphological transition modes in the self-
assembly of block copolymers in solvents
Matrix Composites, Shanghai Key Laboratory of Electrical Insulation and Thermal
Ageing, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240,
P. R. China. E-mail: yfzhou@sjtu.edu.cn; Fax: +86 21 54741297
b College of Material, Chemistry and Chemical Engineering, Hangzhou
Normal University, No. 16 Xuelin Road, Hangzhou 310036, P. R. China.
E-mail: yongjin-li@hznu.edu.cn
(I) One to zero
(II) One to one
A - unimers or A " unimers
½AþBꢀ
½AþBꢀ
A A
B A
B or A
Ð
C or A
B
ƒƒƒ!
½AþBꢀ
Ð
½BþCꢀ
Ð
½AþBꢀ
½BþCꢀ
B
B
C
ƒƒƒ! ƒƒƒ!
† Electronic supplementary information (ESI) available: Details of experiments
and characterizations, and supplementary results. See DOI: 10.1039/c6cc03445d
(III) One to two or more A - B + C or A " B + C
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Scheme 1 Schematic illustration of the overall light-responsive transition
of the supramolecular block copolymer of PEG-b-HBPO.
back into vesicles again under visible light. Since PEG-b-HBPO
can transform into two amphiphiles under the light stimulus, we
define it as a ‘‘latent double amphiphilic’’ supramolecular block
copolymer.
Details about the synthesis and characterizations of linear
amphiphile AZO-PEG and hyperbranched amphiphile CD-g-HBPO
can be found in the ESI.† The CD unit grafted on hyperbranched
HBPO retains the capability to host the AZO groups (Fig. S5, ESI†).
Thus, a linear-hyperbranched supramolecular polymer of PEG-
b-HBPO will be obtained through the AZO/CD complexation
between AZO-PEG and CD-g-HBPO.
The host–guest complexation of AZO-PEG and CD-g-HBPO
(molar ratio of AZO: CD = 1 :1) was induced by adding water
dropwise into a DMF solution of them. Dynamic light scattering
(DLS) measurements showed a unimodal distribution (Fig. 1a)
and a continuous increase of hydrodynamic diameter (Dh) for the
aggregates in solution (Fig. 1b) with a volume percentage ratio of
water/DMF increasing from 0% to 35%. The change of Dh for the
aggregates in Fig. 1b can be divided into two stages.4a In the first
Fig. 1 (a) DLS results of AZO-PEG and CD-g-HBPO mixed solutions with
different water/DMF volume percentage ratios (v/v). (b) Changes in Dh as a
function of water/DMF volume percentage ratios. Insets show the Tyndall
scattering of mixed solutions. (c) UV-vis spectra of PEG-b-HBPO aqueous
solution with different UV irradiation durations (l = 365 nm). Inset shows
the magnification of curves at 400–450 nm. (d) Reversible changes of
absorbance at l = 325 nm under recycling irradiation of UV and visible light
for 24 hours. Insets show photos before and after the vis-UV cycles. (e) DLS
plots of PEG-b-HBPO aqueous solution with different UV irradiation durations.
(f) The DLS plots of individual AZO-PEG and CD-g-HPBO aqueous solution,
and their mixtures at different times.
stage (water/DMF: 0–15%), Dh was increased slightly from 3.1 nm ruptured after freeze-drying under vacuum for a week (insets
to 5.3 nm. AZO-PEG and CD-g-HBPO are both quite soluble in in Fig. 2a) show their hollow interior, indicating a vesicular
DMF, and thus the Dh of 3.1 nm is supposed to represent the size structure. The transmission electron microscopy (TEM) image
of unimolecular CD-g-HBPOs, which is also supported by the of the samples stained with uranyl acetate clearly shows the
calculation results (ESI†). The Dh of 5.3 nm is equal to the particles possess a structure of an inner pool and an outer black
calculated size of the supramolecular polymer of PEG-b-HBPOs thin wall, which further supports that they are unilamellar
(Fig. S7, ESI†), which supports the 1: 1 host–guest complexation vesicles (Fig. 2b and ESI†). In addition, some intermediates
of AZO-PEG and CD-g-HBPO at the water/DMF volume percentage of particles with holes in their membrane, and particles in
ratio of 15%. In the second stage (water/DMF: 15–35%), a rapid budding or fusion during the TEM observations further support
increase of Dh up to 530 nm was observed, which was attributed that the self-assemblies are vesicles (Fig. S9a and b, ESI†). The
to the self-assembly behaviour of the supramolecular PEG-b- thickness of the vesicle membranes is around 10 nm according
HBPOs. Meanwhile the solution turbidity and Tyndall scattering to the TEM (Fig. 2b and Fig. S9c, ESI†), and atomic force
increased gradually, indicating the formation of supramolecular microscopic (AFM) (Fig. S9d, ESI†) measurements. Since the
aggregates as well.
molecular size of PEG-b-HBPO is around 5 nm estimated from
The supramolecular aggregates at a water/DMF ratio of 35% the DLS result as well as the calculation (Fig. S7, ESI†), the
(v/v) were dialyzed against water to remove DMF and characterized vesicles are supposed to possess a bilayer structure. In addition,
by size and morphology. The DLS results show that the aggregates the vesicles have a core shell structure consisting of hydropho-
have a narrow size distribution with a PDI of 0.013, and the final bic HBPO cores and hydrophilic PEG shells according to the
Dh was kept at around 600 nm (Fig. S8, ESI†).
1H NMR measurements (Fig. S10, ESI†). Thus, the packing
The scanning electron microscopy (SEM) image (Fig. 2a) sug- model of PEG-b-HBPOs in the vesicles can be summarized as
gests the aggregates are spherical particles, and some particles shown in Scheme 1.
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corresponding to the p–p* transition of the trans-AZO groups, is
obviously decreased; while the absorbance peak at l = 424 nm,
corresponding to the n–p* transition of cis-AZO groups, is
increased. Such a result was further supported by a quantitative
analysis of the UV-triggered isomerization process (Fig. S12, ESI†).
When exposed to visible light for 24 h, the cis-AZO groups were
transferred back to the trans-form again, accompanied with an
increase of the absorbance peak for trans-AZO at l = 325 nm
(Fig. 1d). Furthermore, the absorbance peak for trans-AZO was
decreased again when the vesicle solution was further exposed to
UV irradiation for another 24 h. This process is reversible and can
be repeated for several cycles. Herein, a duration of 24 h was
selected for the UV and Vis irradiation in order to ensure the
complete light-responsive isomerization for the AZO groups.
The disassembly process during the trans-to-cis transition was
also monitored. Firstly, a phenomenon should be noticed that
after UV irradiation, the solution still exhibited Tyndall scattering,
indicating that there were still nanoscaled objects present.
Secondly, as shown in the DLS measurements, before UV irradia-
tion, only one peak for vesicles with a Dh B 600 nm was observed;
after UV irradiation for two hours, the peak at Dh B 600 nm
disappeared and two new peaks appeared; after UV irradiation
for 12 h, two peaks at Dh B 300 nm and Dh 4 1 mm were formed
(Fig. 1e). These two peaks were kept consistent with extending the
irradiation time further. The DLS results might suggest that the
vesicles were disassembled into two other supramolecular struc-
tures under UV light. To prove it, samples were taken out
immediately after UV irradiation for 24 h and then freeze-dried
for SEM and TEM examinations. A mixture of fibrous and sheet-
like aggregates were observed (Fig. 2c and Fig. S13a, ESI†), while
spherical vesicles totally disappeared. Interestingly, after sub-
sequent exposure to visible light for 24 hours, the nanofibers
and nanosheets disappeared and vesicles were re-formed again
(Fig. S13b and c, ESI†).
Fig. 2 (a) SEM image of the PEG-b-HBPO self-assemblies. Insets show
the ruptured vesicles after freeze-drying for a week. (b) The TEM image of
aggregates stained with uranyl acetate. Inset is a magnified particle. (c) SEM
images of aggregates in PEG-b-HBPO aqueous solution after UV irradiation
for 24 hours. (d) SEM images of the CD-g-HPBO nanosheets. (e) TEM images
of the CD-g-HPBO nanosheets. (f) TEM image of AZO-PEG nanofibers
stained with uranyl acetate. Scale bar (a–f) = 5, 1, 1, 0.5, 1, 0.5 mm,
respectively.
In short, the self-assembly of supramolecular PEG-b-HBPOs
The average size of the vesicles is calculated to be 630 ꢁ 90 nm undergoes an unprecedented reversible ‘‘one to two’’ (i.e., ‘‘vesicles
by randomly measuring 123 particles in the SEM images and to nanofibers plus nanosheets’’) morphological transition under
615 ꢁ 72 nm by measuring 88 particles in the TEM images. The UV/vis stimuli. So, what is the mechanism for this special transi-
sizes calculated from the SEM and TEM images agree well with tion? To address it, the disassembly process of PEG-b-HBPOs in a
the DLS data (Dh B 600 nm). Interestingly, these vesicles in a molecular scale should be considered. As mentioned above, PEG-b-
concentrated solution could further aggregate into 3D colloidal HBPOs will be disassociated into AZO-PEGs and CD-g-HBPOs
crystal-like close-packed arrays (Fig. S11, ESI†) during the freeze under UV light. So, the self-assembly behaviours of AZO-PEGs
drying process due to the narrow size distribution. Colloidal and CD-g-HBPOs must be investigated. In fact, both AZO-PEG
crystal-like structures have been widely observed in the self- and CD-g-HBPO are amphiphilic in nature. For AZO-PEG, AZO is
assembly of hard monodisperse colloids, however it has been a hydrophobic head and PEG is a hydrophilic tail; while HBPO is
hardly observed in the self-assembly of vesicles.4a
hydrophobic and CD is hydrophilic in CD-g-HBPO. According to
As we know, AZO groups can reversibly switch their molecular the SEM and TEM measurements, CD-g-HBPOs self-assembled
conformation between cis- and trans-forms in response to irradia- into irregular nanosheets in aqueous solution (Fig. 2d, e and
tion with UV and visible light, respectively. The trans-form matches Fig. S14, ESI†). The nanosheets are not uniform in size and have
the size and hydrophobicity of the b-CD’s cavity, thus forming the a Dh B 300 nm according to the DLS data (Fig. 1f). The height of
host–guest complex, while the cis-form does not. Therefore, the the nanosheets is measured to be 6.1 nm by AFM (Fig. S14c,
supramolecular PEG-b-HBPO is supposed to be disconnected ESI†). Given that the molecule size of CD-g-HBPO is about 3.1 nm
under UV irradiation, leading to the disassembly of the vesicles (Fig. S7, ESI†), almost 1/2 of 6.1 nm, the nanosheets are supposed
as a result. The UV-vis spectrum and DLS results prove this to possess a bilayer structure. The 1H NMR measurement by
deduction. As shown in UV-vis spectrum (Fig. 1c), with an increase adding D2O gradually into DMSO-d6 solution of CD-g-HBPOs
in UV irradiation time, the absorbance peak at l = 325 nm, indicates the nanosheets have a hydrophilic corona of CDs and
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hydrophobic cores of HBPOs (Fig. S15, ESI†). Thus, a possible process from vesicles to nanosheets plus nanofibers. The process is
structure of the nanosheets can be illustrated as shown in reversible under alternating UV and visible light irradiation. It can
Scheme 1. The CD-g-HBPO nanosheets are very stable, and could be induced from the work that the ‘‘latent double-amphiphilic’’ or
be kept without changes in morphology and size for at least half even ‘‘latent multi-amphiphilic’’ supramolecular polymers are
a year.
very promising to generate more complex and multiple-state
AZO-PEGs self-assemble into well-defined long nanofibers of morphology transitions, and then greatly enhance the controll-
about 10 nm in diameter and micrometers in length according to ability on the self-assembly structure and dynamics.
the TEM and AFM measurements (Fig. 2f and Fig. S16, ESI†),
The authors thank the financial support from the National
which agrees well with the results of a similar system reported Basic Research Program (2013CB834506), China National Funds
by Li and coworkers.7 The nanofibers have a Dh greater than 1 mm for Distinguished Young Scholar (21225420), National Natural
in the DLS measurement (Fig. 1f), which has also been observed Science Foundation of China (21474062, 21404070, 91527304)
by others for the micrometer-long nanofibers.8 The nanofibers and Jiangsu Collaborative Innovation Center of Biomedical Func-
are flexible, and they tend to aggregate together to form thicker tional Materials.
nanofibers and even nanofiber balls (Fig. S16, ESI†). The 1H NMR
measurements indicate the hydrophobic AZO groups are in the
fibre core and hydrophilic PEG chains are stretched outside as
the shell (Fig. S17, ESI†). Given that, the AZO-PEG nanofibers
Notes and references
1 (a) G. Chen and M. Jiang, Chem. Soc. Rev., 2011, 40, 2254; (b) Q. Yan
should have a molecular packing model as shown in Scheme 1.
Therefore, CD-g-HBPOs and AZO-PEGs self-assemble into
nanosheets and nanofibers, respectively. Thus, the disassembly
process of PEG-b-HPBOs is much clearer now. The supramole-
cular polymers of PEG-b-HPBOs disassociated into CD-g-HBPOs
and AZO-PEGs under UV irradiation, and then the disassociated
CD-g-HBPOs formed nanosheets and AZO-PEGs formed nano-
fibers, which consists of the UV-triggered ‘‘one to two’’ morpho-
logical transition from PEG-b-HPBO vesicles to CD-g-HBPO
nanosheets and AZO-PEG nanofibers.
To further prove the reversibility, the reverse self-assembly
process by mixing CD-g-HBPO nanosheets and AZO-PEG nano-
fibers together in water was also studied under visible light.
As shown in the DLS results (Fig. 1f), at the beginning there
were two peaks with Dh B 300 nm assigned to the CD-g-HBPO
nanosheets and Dh 4 1 mm assigned to AZO-PEG nanofibers.
Then, with the elapse of time these two peaks gradually disappeared
and a new peak at Dh B 600 nm appeared instead. Further TEM
measurements show that the aggregates with the Dh B 600 nm are
spherical vesicles (Fig. S18, ESI†). In other words, CD-g-HBPO
nanosheets and AZO-PEG nanofibers did transit into vesicles again
under visible light.
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In summary, herein, we report an unusual supramolecular
polymer structure with ‘‘latent double amphiphilicity’’, which
shows a new light-responsive ‘‘one to two’’ morphological transition
Chem. Commun.
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